Dispersive delay lines have been utilized in the past for many purposes such as compression filters which compress signals by making the delay of the signal applied to the line proportional to frequency or alternatively, as a chirp local oscillator which generates an output signal whose frequency rapidly sweeps through a given band. One of the most popular uses for the dispersive delay line is in compressive receivers, in which the presence and frequency of an incoming signal is ascertained by heterodyning the incoming signal with a fast-sweeping variable frequency oscillator signal. The dispersive delay line compresses the resulting signal and its output when sampled at a particular time shows the existence of a signal at a predetermined frequency.
For purposes of this invention a dispersive delay line is a network arranged so that signals at the low end of a given band are delayed .tau..sub.1 seconds, and the signals at the high end of the band are delayed .tau..sub.2 seconds, with a linear relationship in between: ##EQU1## The result is that for a frequency sweep matched to the frequency is delay characteristic of the line, the low frequency part of the signal emerges from the network simultaneously with the high frequency part, and indeed all frequency components of a signal emerge simultaneously.
In the past, there have been a number of different dispersive delay line techniques. One delay line is called a "meander line" and was developed and used by the Stanford Research Institute in their pioneering microscan receiver developments for the U.S. Air Force and U.S. Army. The meander line was a delay line consisting of bridged T microwave sections which were planar in structure. While excellent early results were obtained with the meander line, which included a 600 MHz bandwidth and a 200 nanosecond differential delay, this particular type of line is difficult to fabricate and expensive to manufacture.
More recently, surface acoustic wave (SAW) devices have been utilized in pulse-compression. In 1976, R. D. Weglein et al, of Hughes, reported a 500 MHz bandwidth surface wave pulse compressive filter operating about a center frequency of 1.3 GHz. Mellon and Bell of Texas Instruments described the development of a UHF surface wave pulse compressor, which device was designed to operate between 1.0 and 1.5 GHz. Also in the same year, Williamson et al presented a paper on "L-Band Reflective Array Compressor with a Compression Ratio of 5120." Weglein's line used electron beam fabrication as did that of Mellon and Bell. Williamson's line was a reflective array compressor utilizing ion beam etching for pattern generation.
In most instances, the reflective array compressor lines are fabricated on lithium niobate and BGO. Here, the bandwidth is resolution limited at about 500 MHz.
Limits of high frequency operation of SAW devices, whether they use interdigitated finger patterns, reflective grooves, or combinations of these, depends upon the ability to accurately produce the pattern to be manufactured on the required substrate. At the present time, SAW State-of-the-Art technology appears to be progressing towards a practical 1 GHz bandwidth filter.
One family of dispersive delay lines not heretofore mentioned, are the magnetostatic wave delay lines. The principle of operation of these delay lines depends on continuous, inherent dispersive properties of the delay medium that potentially eliminates the need for high resolution transducer patterns. At the same time, the propagation velocity is low enough to design small devices. Both magnetostatic surface wave results and bulk wave results show promise of very high frequency operation from several GHz to tens of GHz. However, these particular lines have excessive insertion loss and an excessive latency time for purposes of high performance pulse compression receivers.
As compared to the State-of-the-Art compressive filters, a compressive filter can by the present teaching be fabricated from a simple conventional "semi-rigid" 50 ohm coaxial cable, in which the cable is provided with discontinuities at predetermined spaced intervals, which discontinuities reflect energy of a predetermined frequency back through the line by virtue of the spacing between the discontinuities. In one embodiment this is accomplished by a technique in which the outer conductor is crimped inwardly at selected points to provide a single port device which has a wider bandwidth than the SAW devices with lower insertion loss. When the axial crimp length is small compared to the wavelength, the crimped portion behaves as a simple capacitive shunt to reflect incoming energy back down the line, selectively by frequency as a function of crimp spacings. For purposes of this invention "crimping" refers to the compressing or pinching of the outer conductor of the coaxial line towards the inner conductor. In conventional "semi-rigid" coaxial lines the outer conductor is copper or steel.
At the present time, with the crimp coax technique, it is possible to obtain a demonstrable pulse compression bandwidth of up to 3.6 GHz and low insertion loss in a single line. Other features are a single port, 50 ohm impedance and insensitivity to temperature changes, and further automatic temperature compensation by forming the linear-FM swept local oscillator by impulsing a crimped line of similar design. The crimping is accomplished by a crimping tool which in one embodiment has four teeth which squeeze into the line to provide the capacitative shunt. Alternatively, the line may be compressed at the required point by parallel spaced apart blades in a scissors-like pinch action, or by a pipe-cutting type tool which crimps the line symmetrically into an annular groove.
It should be noted that the use of a coaxial cable as a delay line is not without precedent. The simple, reliable wideband "uncrimped" coaxial cable delay line has been in use for over twenty years in many thousands of repeater jammers as a microwave memory element. No other device (bulk acoustic, SAW or meander line) has proven as economical in this application when large bandwidth and low loss are required.
It is, therefore, the purpose of this invention to take an extremely reliable mass produced product, whose properties are well known and provide reflective, crimped coax capacitive shunts therein so as to provide a wide bandwidth dispersive delay line which is compact and low cost. The crimped coax dispersive delay line is a single port device which may be easily and inexpensively mass produced with a precisely controlled characteristic impedance of 50 ohms thus requiring no matching networks, and with a low temperature coefficient of delay which eliminates the need for temperature control. Independent crimping for amplitude weighting and linear time delay corrections is accomplished within the line and the compact structure needs no shielding or special packaging.
It should be noted, moreover, that the subject dispersive delay line may be used in one or both of two modes: (1) as a compressive filter for use with compressive receivers and (2) as a chirp local oscillator which, when fed with a short impulse signal effectively having all frequency components, produces sequentially signals differing in frequency so as to provide a sweep or chirp local oscillator signal at its output.
It will be appreciated that the subject invention has been described in terms of a dispersive delay line in which the delay is a linear function of frequency. The crimped coaxial line may also be configured to function as a bandpass filter. In one embodiment, with equally spaced crimps, the filter has a ##EQU2## characteristic. Alternatively, with appropriate amplitude and phase weighting, the line can be tailored to a desired bandpass characteristic involving a predetermined sidelobe structure. It will be appreciated that the amplitude weighting is a function of the depth of the crimp, and the phase is a function of crimp positioning. It should be noted that bandpass filters have been made in the past from waveguides provided with spaced stubs. This is however an expensive process and the crimped coaxial line offers significant cost advantages as well as certain performance advantages. Also due to the flexibility of the crimped coaxial line, packaging dimensions can be minimized with the coiling of the lines.
It is, therefore, an object of this invention to provide a unique, dispersive delay line with wide bandwidth and which is simply and easily fabricated from standard, coaxial cable.
It is an object of this invention to provide a unique bandpass filter made by crimping standard coaxial cable.
It is another object of this invention to provide a dispersive delay line in which inexpensive crimping is utilized to give the coaxial line a dispersive characteristic.
It is another object of this invention to provide a method of fabricating a dispersive delay line which involves the crimping of conventional, coaxial cable.
It is a yet still further object of this invention to provide a dispersive delay line by providing a series of discontinuities in a conventional coaxial cable.
These and other objects will be better understood when viewed in light of the following description taken in conjunction with the following drawings :